1. Field of the Invention
The present invention relates generally to light emitting devices, and, more particularly, to a system and method for the monolithic integration of a light emitting device and a photodetector that uses a native oxide semiconductor layer to realize low spontaneous emission capture and low bias voltage operation.
2. Related Art
Semiconductor lasers in general and vertical cavity surface emitting lasers (VCSELs) in particular are used for many applications including electronics, communication systems, and computer systems. Lasers produce light that can be transmitted directionally. In many applications of lasers, and particularly in many VCSEL applications, there is a need to precisely control the laser output power. The output power of semiconductor lasers is primarily determined by the bias current. However, it can be significantly altered by the ambient temperature and aging of the device. For this reason, control of the output power is realized by monitoring the laser output and adjusting the laser current to maintain a specified laser output power. The light measurement is typically performed using a semiconductor photodetector, while the feedback loop is realized using an external electronic circuit. There are numerous implementations of such laserphotodetector systems, and they differ in application and performance.
The two primary design issues relating to the laser-photodetector system are the cost of the device and the ability to provide performance required for a specific application. From a cost perspective, it is desirable to build the laser and the photodetector on the same chip using the same or similar fabrication technology. This is realized by monolithic integration of the laser and the photodetector. Monolithic integration implies that the individual laser and photodetector devices are completed jointly at the wafer level. From a performance perspective, there are a number of desired qualities.
An important issue for efficient laser output power adjustment is the tracking between the detector current and the laser directional output power. There is no way to distinguish between the fraction of the detector current generated by the directional laser output and the fraction generated by spontaneous emission. Since the external feedback circuit requires information relating to the directional laser output power, the fraction of the detector current generated by spontaneous emission can be a source of error. This error can be minimized by minimizing the spontaneous emission captured by the detector. In addition, for efficient operation of the external feedback loop, the tracking between the directional laser output power and the photodetector current should be stable and repeatable.
Another important issue is the electrical interaction between the photodetector and the laser. The existence of the photodetector and its biasing should have a negligible effect on the operation of, and in particular, the modulation properties of the laser. The laser modulation and biasing should also have a negligible effect on the operation of the photodetector. This implies that the laser and the photodetector should be electrically de-coupled and exhibit negligible high frequency cross-talk.
Lastly, the incorporation of the laser-photodetector device into the external driver circuit should be considered. In computer communications applications the minimum bias voltage is an issue of increasing importance due to the desirability of reduced computer power consumption. Today""s computer architectures are using 3.3 volt (V) power supplies having a lower limit of approximately 3.1 V. In the future and for other applications, it is foreseeable that the power dissipation will be reduced even further requiring even lower bias voltage levels.
A preferable configuration of the laser-photodetector system is one in which the laser and photodetector are independently biased from the same power supply. In order to achieve this result, the power supply voltage must be larger than the laser operating voltage, which depends on the photon energy, and the photodetector operating voltage, which depends upon the photodetector reverse bias required for efficient performance. For optical communications, the vertical cavity laser voltages range between approximately one to two volts, while the typical photodetector reverse bias voltage is between 0.5 and 1 volt. For other applications these voltages may vary.
In order to use the lowest power supply voltage possible an integrated laser and photodetector configuration enables forward biasing the laser and reverse biasing the photodetector from the same power supply. This is always achievable by using a four terminal device structure in which the laser and the photodetector can be independently biased with arbitrary polarity. If the structure permits, two of the four terminals can also be jointly connected to one power supply thereby creating a three terminal device in which the same power supply simultaneously forward-biases the laser and reverse-biases the photodetector. The realization of three terminal devices that allow this preferred biasing scheme is not always possible due to fabrication and structural limitations.
In the past, photodetectors have been integrated with lasers to varying degrees of success. For example, some integration schemes use a photodetector and laser that have been independently fabricated on different chips. The two devices are integrated at the packaging stage, after fabrication, resulting in arbitrary relative polarity between the laser and photodetector. This integration scheme is referred to as xe2x80x9chybrid integrationxe2x80x9d. The primary disadvantage of this approach is that the extra processing step of integrating the photodetector with the laser after fabrication undesirably adds manufacturing cost. Additionally, in many cases the relationship between the photodetector current and the laser output is neither stable nor repeatable, due to the fluctuation in the laser output beam shape.
Another scheme involves monolithic integration of a photodetector and laser where the coupling is realized using side emission, resulting in both three or four terminal devices. The main disadvantage of such devices is that the photodetector does not detect the directional laser output, but predominately captures the omni-directional spontaneous emission.
Finally, another scheme involves the monolithic integration of a laser and a photodetector where the coupling is realized by top (or bottom) emission, resulting in both three and four terminal devices.
All of the implementations result in either three terminal devices where the laser and the photodetector share a common n-side (cathode) or a common p-side (anode), which requires a relatively high bias voltage for operation and in which the laser and the photodiode are electrically coupled; or in four terminal devices in which the photodetector captures a high proportion of spontaneous emission (SE) from the laser. Therefore, a monolithically-integrated laser and photodetector device that can operate at a low bias voltage, enables electrical de-coupling between the laser and the photodetector, and which minimizes the capture by the photodetector of omni-directional spontaneous emission from the laser is desired.
FIG. 1A shows a prior art laser and photodetector combination in a three terminal configuration in which an unacceptably high level of spontaneous emission is allowed to reach and be detected by the photodetector. Laser and photodetector 11 is essentially comprised of photodetector 12 deposited over laser 13 in a common cathode arrangement. The common cathode configuration is also referred to as PNP configuration because the semiconductor conductivity type changes twice in the structure. Laser 13 is typically a vertical cavity surface emitting laser (VCSEL). This arrangement is illustratively characterized as having two pn junctions. The first pn junction is active layer 14 located within laser 13 and the second pn junction is the absorbing layer 16 within photodetector 12. The laser 13 comprises a p-type substrate 22 on the bottom of which a p-type contact layer 21 is deposited. Over the substrate 22 is p-type mirror 23. Active region 14, which includes an n-type material and a p-type material separated by a light generating medium is grown over p-type mirror 23. Over active region 14 is n-type mirror 24, over which is grown n-type contact material 26.
Immediately upon n-type mirror 24 is n-type layer 31 of photodetector 12, over which absorbing layer 16 and p-type layer 32 are grown. Layers 32, 16, and 31 comprise a photodetector in a pin configuration. Over the p-type material 32 is p-type contact material 33.
In a typical VCSEL there are two components to the optical power within the structure. Below the lasing threshold, the light output consists of spontaneous emission which increases with the bias current. Spontaneous emission is spectrally broadband and generally omni-directional. At threshold and at bias currents above threshold, the spontaneous emission intensity saturates and the lasing modes appear and quickly dominate the light output. Both the wavelength emission spectrum and the angular distribution of the lasing modes are generally much narrower than those of the spontaneous emission. Because the spontaneous emission has such a wide angular distribution, a large fraction of that light never leaves the device because it is totally internally reflected at the large refractive index discontinuity between the top semiconductor layer and air, or at the semiconductor to air interface at the bottom surface of the device.
The light at large angles of incidence is totally reflected because the refractive index of the medium from which the light is incident, in this case semiconductor, is larger than the surrounding medium, in this example air. The lasing modes, on the other hand, are more directional, having components that are at most at a small angle with respect to the surface normal and are thus largely transmitted through the semiconductor to air interface.
Still referring to FIG. 1A an integrated photodetector 12 will capture the large angle spontaneous emission rays because there is no layer with a large refractive index difference between the VCSEL 13 and the photodetector 12 that would produce a total internal reflection of the spontaneous emission from the laser incident at large angles. For this reason the detector absorbs a large fraction of the omni-directional spontaneous emission in addition to absorbing the directional lasing modes.
FIG. 1B is a graphical representation 41 of the prior art laser and photodetector of FIG. 1A including example VCSEL light output and photodetector monitor current associated therewith. The horizontal axis of graph 41 indicates laser drive current and the vertical axis indicates VCSEL light output represented by line 44, and monitor photodetector current output represented by line 42. As can be seen the VCSEL directional light output below lasing threshold 51 as illustrated by portion 48 of line 44 is quite low. The directional laser output here is the light intensity that would be measured if the light was coupled into an optical fiber or measured using a detector external to the laser.
In contrast thereto, the monitor photodetector current output, which should be an indicator of detected output from the laser, shows significant response due to captured spontaneous emission below lasing threshold 51. This is illustrated by portion 47 of line 42. Because the measured monitor photodiode current does not exactly correspond to the laser output intensity, this condition is very undesirable. It is difficult for the integrated detector, and hence the external feedback circuit which measures the detector current, to resolve what fraction of the detected light is attributable to the directional laser output rather than to spontaneous emission. Therefore, high spontaneous emission capture by the integrated monitor photodiode degrades the ability of the external feedback circuit to effectively adjust the directional laser output power. The adjustment of the target directional output power cannot be performed efficiently unless the spontaneous capture is negligible when compared to the directional output power.
FIG. 1C is a schematic view illustrating exemplary driver circuitry associated with the prior art laser and photodetector of FIG. 1A. Driver 61 is represented with an NPN bipolar transistor, while the photodetector (PD) current amplifier is represented as current mirror 62 using NPN bipolar transistors 64 and 66. The actual circuit may vary with the essential elements represented herein as an example. The integrated VCSEL and photodetector device 11 is shown in a common cathode configuration, also referred to as a PNP configuration. There are several disadvantages to this structure. First, with typical operating voltages of 2 V on the VCSEL, represented by VL 67, 0.5V on the PD, represented by VPD 68, and 0.8 V on the bipolar transistor 61, there is insufficient bias headroom to use a power supply with a 3.1 V minimum voltage. This condition could be resolved were the VCSEL and PD electrically de-coupled, which would enable the same power supply to simultaneously forward bias the VCSEL and reverse bias the PD.
Second, in this structure the modulation and biasing of the VCSEL must be done from terminal 69 between the VCSEL and the PD. Terminal 69 is the terminal to which the collector of transistor 61 is connected. This arrangement implies that the parasitic capacitance of the PD loads the VCSEL driver 61, and that the adjustment of the VCSEL operating point inadvertently changes the operating point of the PD. Hence, the common cathode, or common anode, configuration, which uses the same power supply to simultaneously forward bias the laser and reverse bias the photodetector prevents electrical de-coupling of the VCSEL and the PD.
Thus, an unaddressed need exists in the industry for a monolithically integrated light emitting device and photodetector arrangement that operates using a low bias voltage, that provides electrical de-coupling between the light emitting device and the photodetector, and that minimizes the amount of spontaneous emission detected by the photodetector.
The invention provides a light emitting device and photodetector in a monolithically-integrated structure which enables operating bias voltages that are lower than previously achievable, that provides electrical de-coupling between the light emitting device and the photodetector, and that minimizes the amount of spontaneous emission detected by the photodetector. Although not limited to these particular applications, the system and method of the present invention are particularly suited for monolithically integrating a photodetector and a vertical cavity surface emitting laser (VCSEL) in a novel configuration that minimizes the transmission of omni-directional spontaneous light emission from a laser to a photodetector. The system and method for the monolithic integration of a light emitting device and photodetector using a native oxide semiconductor layer can be implemented using a variety of epitaxially grown semiconductor materials having various electrical properties. For example, the material layers to be described hereafter in a preferred and several alternative embodiments can be of either n-type or p-type material without departing from the concepts of the invention.
In architecture, the present invention can be conceptualized as a system for measuring the output of a light emitting device comprising a light emitting device having a light emitting device refractive index optically coupled to a photodetector having a photodetector refractive index. Moreover, a continuous insulating layer having a refractive index lower than that of both the light emitting device refractive index and the photodetector refractive index contacts the light emitting device and the photodetector.
In an alternative embodiment of the monolithically integrated light emitting device and photodetector, a Schottky photodetector is incorporated into the structure.
In another alternative embodiment of the monolithically integrated light emitting device and photodetector, a metal semiconductor metal Schottky photodetector is incorporated into the structure.
In yet another embodiment of the monolithically integrated light emitting device and photodetector, the photodetector and the continuous insulating layer are formed within one of the mirrors of the light emitting device.
The present invention may also be conceptualized as providing a method for constructing a monolithically integrated light emitting device and photodetector comprising the following steps.
A light emitting device having a light emitting device refractive index is formed. A photodetector having a photodetector refractive index is incorporated with the light emitting device. A continuous insulating layer having a refractive index lower than that of both the light emitting device refractive index and the photodetector refractive index is located at a junction of the light emitting device and the photodetector.
The invention has numerous advantages, a few of which are delineated, hereafter, as merely examples.
An advantage of the invention is that it allows a photodetector to track the directional light output from a light emitting device, while minimizing the capture of omnidirectional spontaneous emission from the light emitting device.
Another advantage of the invention is that the photodetector and the light emitting device are electrically de-coupled, thereby enabling independent biasing of the light emitting device and the photodetector, and eliminating the unnecessary loading of the light emitting device driver circuit by the photodetector parasitic capacitance. This arrangement improves the high-frequency response of the device.
Another advantage of the present invention is that it lends itself to simple integration into arrays of devices in which each detector and light emitting device may be individually biased in a straightforward manner. This arrangement allows the integration of a photodetector on arrays of VCSELs fabricated on a common conductive substrate due to the laser and photodetector being electrically isolated.
Another advantage of the invention is that it is simple in design and easily implemented on a mass scale for commercial production.
Other features and advantages of the invention will become apparent to one with skill in the art upon examination of the following drawings and detailed description. These additional features and advantages are intended to be included herein within the scope of the present invention.